Optical instruments which measure target reflectance are employed to indirectly determine diverse characteristics of targets within the field of view. For example, the device set forth in U.S. Pat. No. 3,598,994, upon which is based the famous Fraunhofer Line Discriminator (FLD) used for some years by the United States Geological Survey at Flagstaff, Ariz., measures, separately, (a) reflectance within and (b) apparent reflectance adjacent a selected Fraunhofer band to obtain a measurement of luminescence in the Fraunhofer band issuing from a target under stimulation by direct sunlight. A handheld optical instrument for obtaining like measurements, but employing a different technological implementation approach, is disclosed in copending U.S. patent application Ser. No. 509,681, filed May 30, 1983, by Robert D. Watson for "Portable Luminescence Sensor". Certain observation and meteorological satellite systems measure upwelling radiances at several wavelengths and telemeter the data to ground stations. The measurements obtained with these instruments and systems may be analyzed to classify target spectral characteristics from which mineralogical, geological, and other target information can be deduced, often in conjunction with target characteristics obtained from still other measurements taken with other instruments.
In order to achieve the necessary measurement accuracy, those skilled in the art will appreciate that instruments of the contemplated type which are carried by aircraft or by hand must be very closely calibrated, not only in the laboratory, but also in the field. Such calibration requires the use of a standard target illuminated by (at least in the laboratory) a standard light source directed onto the target from which a predetermined measurement by the instrument under proof should be obtained. Any deviation from the expected measurement may be used to recalibrate the instrument, derive coefficients involved in the equations for reflectance and luminescence, and/or develop correction factors to be applied to the instrument readings taken in the field. Once a close laboratory calibration has been achieved, it is possible to readjust (or revise the correction factors applied to) an instrument in the field if a reliable standard test target is available.
Both standard illuminants and standard reflectance test targets are well known. Illuminants generally contain energy at many wavelengths, therefore, they must be considered in terms of an energy distribution curve showing the intensity at each wavelength rather than in terms of a single number. Different spectral distributions give a different appearance to the same object. To standardize this variable, several reference illuminants have been established by the CIE (Commission Internationale de l'Eclairage), and the spectral distributions of these illuminants have been accurately determined and published. By way of example, a tungsten filament lamp, operating with a specified voltage and current, will have a color temperature of 2854 K and possesses known energy at each wavelength. This illuminant is identified as the standard CIE Illuminant A which possesses known energy at each wavelength.
It is desirable, usually even necessary, for reflectance standards to have surfaces exhibiting a near Lambertian characteristic. Lambert defined a perfectly diffuse surface (commonly designated as a Lambertian surface) as one for which the radiance is constant for any angle of reflection to the surface normal. For purposes of simplification, the term "Lambertian surface", as used herein, shall, for practical purposes, actually mean a nearLambertian surface. The most commonly used reflectance standards exhibiting Lambertian surfaces are painted barium sulfate (BaSO.sub.4) test plates which may have been, in turn, calibrated against a pressed BaSO.sub.4 or "Halon" (registered trademark of the Eastman-Kodak Co.) powder disc (or another known standard which may have been certified by the National Bureau of Standards) to refine the known accuracy of a given specimen. Painted BaSO.sub.4 plates have two distinct drawbacks which are notoriously well known in the art; viz.: they are very delicate and they are costly. "Halon" discs (which may be obtained from Eastman-Kodak) are not appropriate for field use because they can only be obtained in a two-inch diameter size.